1. Field
The disclosed concept pertains generally to engine-generators and, more particularly, to systems including a plurality of parallel engine-generators. The disclosed concept also pertains to methods of a paralleling a plurality of engine-generators.
2. Background Information
It is desirable to place two or more engine-generators in parallel with each other on a common electrical bus as quickly as possible. Success in this endeavor offers many advantages. Among these advantages is the fact that the National Fire Protection Agency (NFPA), in NFPA 110, Standards for Emergency and Standby Power Systems, and in NFPA 99, Standards for Health Care Facilities, impose a legal requirement for engine-generator (or gen-set) systems to provide emergency back-up power to certain equipment within 10 seconds.
With the time that it takes a gen-set to start and reach its rated voltage and frequency varying from about 5 to 7 seconds, it is believed to be virtually impossible to guarantee that multiple gen-sets can be closed to a common bus, such as an emergency bus, in less than 10 seconds when using conventional paralleling techniques.
A conventional “dead field paralleling” technique consists turning off gen-set excitation fields, closing multiple gen-set circuit breakers to a common bus, starting the gen-sets, and as they approach rated frequency (e.g., without limitation, 60 Hz) and rated voltage, turning on the gen-set excitation fields. When the excitation fields of these gen-sets are energized, electro-magnetic forces pull the gen-sets into parallel with each other. There are several distinct disadvantages to this technique. First, the mechanical forces and stresses exerted on the engines and generators subject to this type of dead field paralleling are relatively very large. Such forces and stresses can significantly reduce the effective life of the gen-sets or damage the generators and/or engines. Second, the relatively large currents that pass between gen-sets during this type of dead field paralleling can exceed the maximum acceptable gen-set current, thereby causing the gen-set circuit breakers to trip and rendering the gen-sets out of commission.
Automatic voltage regulators (AVRs) excite generator fields, thereby controlling the alternating current output voltage of a generator. When engine-generators (or gen-sets) are paralleled to a common bus and are out of phase, the voltage difference between the gen-sets causes current to flow between them. The magnitude of current is a function of AVR field excitation and how far out of phase the gen-sets are.
As shown in FIG. 1, the AVRs 2,4 fully excite the respective gen-sets 6,8. Here, in this example, the gen-sets 6,8 are 180 degrees out of phase. The magnitude of the current 11 flowing between the gen-sets 6,8 (and through the circuit breakers 10,12 and the common bus 14) causes mechanical stresses to the gen-sets 6,8 and sufficient current to trip the generator circuit breakers 10,12.
There are various other prior proposals to synchronize generators. In one approach, the generators are started, the circuit breakers are closed as the generators approach rated RPM (e.g., without limitation, typically 1800), and then the voltage regulators are turned on. However, in this approach, stresses to the engine and the generator are relatively very high and the current flowing between the generators may cause a generator circuit breaker trip.
For another prior proposal, the voltage regulators are turned off, the generators are started, and the circuit breakers are closed at crank terminate (e.g., when the engine starter motors are disengaged). If a generator is between 600 and 900 rpm, then the corresponding voltage regulator is turned on. If a generator does not reach crank terminate or does not meet the 600 to 900 rpm range, then it is temporarily locked out and the corresponding generator circuit breaker is opened, thereby allowing other generators to move to rated speed (e.g., without limitation, typically 1800 RPM). After the first generator closes to the common bus, then all subsequent generators are allowed to use normal synchronizing methods.
There is room for improvement in systems of paralleling engine-generators on a common bus.
There is also room for improvement in methods of paralleling engine-generators on a common bus.